Liquid crystal display (LED) devices have the advantages of being light weight, thin, and consuming low power, and as a result LCDs have been used for display devices for compact information processing terminals, such as mobile phones, electric dictionaries, etc. Since those compact information processing terminals are usually driven by batteries, it is quite important to reduce power consumption from a view point of making their operation time longer. In the case of a mobile phone, for instance, its power consumption must be as little as possible, at least in the standby state. As a method to comply with such a requirement, Japanese Patent Application Tokkaihei 58-23091 discloses an image display device with a digital memory provided for each pixel to hold a video signal. In this device, a significant reduction of power consumption can be made by suspending the operation of peripheral driving circuits except a control circuit to control the polarity of video signals supplied from the digital memory to pixel electrodes.
Meanwhile, mobile phones have been equipped with color halftone and moving picture display devices for internet, TV phone, etc. and further require high definition display devices with lower power consumption. In order to meet such requirements, a liquid crystal display device has been proposed to provide each pixel with a switch for selecting either one of two modes of operation: a normal display mode using ordinary thin film transistors and a still picture display mode using a digital memory. In this liquid crystal display device, where an area per pixel is made small to achieve high definition display, the digital memory provided for each pixel is necessarily so small in size that the digital memory restricts the driving capability of each pixel. In the case of such a restriction, it is quite difficult to secure a sufficient tolerance for dispersion of device characteristics depending on the device production process. Where the driving capability of a digital memory is lower than its designed values with respect to an electric capacitor of a liquid crystal and its auxiliary capacitor, a point defect takes place at a pixel mistakenly driven by that digital memory in the still picture display mode. This results in a lower yield rate of the liquid crystal display device.